Optical coefficient measurement of spinal cord tissue and numerical simulation for interstitial photodynamic therapy

Surgical excision remains the mainstream therapy for malignant spinal cord tumors, yet neurological morbidity and incomplete resection continue to limit outcomes. Interstitial photodynamic therapy (iPDT) is a minimally invasive local approach in which light is delivered directly into tumor tissue after administration of a photosensitizer. Here, irradiation conditions for iPDT were examined using numerical simulations driven by experimentally obtained optical properties of porcine spinal cord tissue. A three-dimensional spinal cord model with a centrally embedded ellipsoidal tumor (20mm in length and 5mm in diameter) was created, and lateral-emission probes were placed within the target. Ray tracing simulation at 664nm and an irradiance of 150mW/cm² was performed for irradiation durations of 180, 810, and 2400s, followed by heat conduction simulation to estimate temperature changes. The simulations indicated that a three-probe arrangement with 8mm spacing can provide therapeutically relevant intratumoral light coverage while keeping temperature elevation in surrounding tissue limited.